if I change plans on the exede portal does it commit me to a new 24-month minimum term of service? currently on month-to-month

No, Changing plans does not restart your two year contract...You can change plans as often as you wish, if offered on your beam, but must pay for your new plan selection for one month before changing plans again...

Wm4Bama is correct. you can freely move about plans during your term. And the prorated bills are a major consideration as changing will either result in a prorated charge or credit the next month's bill